Vacuum packed (VP) and modified atmosphere packed (MAP) chilled food products generally have the following common characteristics:                they tend to be subjected to minimal heat treatment or pasteurisation to reduce the microbial population in the product. This is not the same as sterilisation. Sterilisation essentially annihilates all micro-organisms in a product;        they are generally vacuum packed or otherwise flushed with an inert gas to completely or partially remove oxygen. This is done to prevent growth of aerobic organisms that are not affected by the pasteurisation;        they are generally stored, delivered and sold under refrigeration conditions (from 0 to 10° C.) to prevent spoilage by mesophilic organisms not affected by the pasteurisation.        
As a consequence of the above processing, VP/MAP products tend to have a shelf life of about 10 days. After this point they succumb to microbial spoilage and have the potential to become unsafe because, as noted above, they are not completely sterilised by the pasteurisation process.
VP/MAP foods tend to have enhanced organoleptic properties and nutritional qualities. This is a key benefit of these foods and it is realised because the foods are pasteurised and not sterilised. More aggressive anti-microbial treatments, such as sterilisation tend to destroy organoleptic properties and nutritional qualities. However, there is one problem that is particular to VP/MAP foods. This is that the anaerobic conditions and refrigeration conditions that are required to prevent spoilage by anaerobic mesophilic organisms still permit germination of psychrotrophic spores of anaerobic bacteria and toxin production therefrom. These spores tend to be ubiquitous in nature and hence can readily contaminate food during manufacture. They also tend to be heat resistant and hence are not readily affected by the pasteurisation treatments applied to VP/MAP foods that allow the enhanced organoleptic properties and nutritional qualities of these foods to be realised.
One example of these spores is the spore of non-proteolytic Clostridium botulinum type E. These spores can germinate and produce toxin at temperatures greater than 3.3° C. i.e. within the range of refrigeration conditions noted above.
This problem is further compounded by the fact that VP/MAP food products are intended to have a shelf-life of up to and beyond 10 days. More specifically, each day that the product is stored in refrigeration conditions increases the likelihood of there being outgrowth of and toxin production by a psychrotrophic spore.
There are a number of ways of minimising the risk of germination of psychrotrophic spores. The regulatory authorities of many countries including the UK and the US require the VP/MAP food product to be subjected to either:
(i) a process wherein the product is exposed to temperatures of 90° C. for 10 minutes; or
(ii) a process of “equivalent lethality”—i.e. a process that provides for the destruction of the same number of psychrotrophic spores in the product as that of exposing the product to 90° C. for 10 minutes.
At the time of the invention, it was generally understood that to be considered a process of “equivalent lethality”, the process must provide for a 6-log reduction in numbers of Type E spores (see: Draft Guidance on the safety and shelf-life of vacuum and modified atmosphere packed chilled foods, UK Food Standards Agency, 2004) US. Food & Drug Administration Center for Food Safety & Applied Nutrition FISH AND FISHERIES PRODUCTS HAZARDS AND CONTROLS GUIDANCE Third Edition June 2001, chapter 17). At this time it was understood that this level of spore reduction would invariably require a process including a heat treatment step of greater than 80° C. Further such heat treatment was generally understood as tending to result in loss of organoleptic properties and nutritional quality of foods.
Where such a heat treatment is not applied (for example, because of concern of loss of organoleptic properties and nutritional quality), a milder heat treatment step (for example less than 80° C.) tends to be used and combined with a further step of adjusting salt, acidity and/or water activity, or adding preservatives such as nitrite. Clearly, these adjustments may also affect the organoleptic properties and nutritional quality of VP/MAP food products.
There is a need for a process for pasteurisation and production of VP/MAP food products that achieves a destruction of spores, especially non-proteolytic C. botulinum type E spores, that is equivalent to that achieved by heating the product at 90° C. for 10 minutes and that, in particular, does not substantially affect the organoleptic properties or nutritional quality of the food product.